1. Field of the Invention
The present invention relates to a monitoring device for an injection molding machine, and more particularly, to a monitoring device capable of monitoring heat generated due to shearing of resin.
2. Description of Related Art
In metering process of an injection molding machine, an injection cylinder is heated by a heater and metering is carried out by retracting a screw while at the same time rotating the screw to plasticize, melt, and knead resin in the injection cylinder. At this time, resin pellets are plasticized and melted by shearing heat generated due to shearing of the resin pellets by the rotating screw as well as by heat generated by the heater.
Resins used for optical components such as lenses, however, are deteriorated by shearing. It is therefore desirable that certain kinds of resins should be melted by heat generated by the heater.
When resin is sheared, plasticized, and melted by rotating the screw, the shearing and plasticization of the resin act upon the screw as a load and thus can be estimated from the magnitude of driving torque of a motor etc. for rotating the screw. There has been known a product quality determination method wherein the driving torque of the motor for rotating the screw is monitored and if the driving torque deviates from a preset allowable range, the deviation is detected as metering abnormality (see, for example, JP 06-297532A).
Also, there has been proposed a control method wherein a torque model for the metering motor, according to which a non-defective product could be produced, is obtained in advance and the current supply to the heater for heating the heating cylinder is controlled such that the metering motor torque approaches the torque model (see, for example, JP 2001-260193A).
Further, a method is known wherein the correspondence between heat generation by the heater, calculated from the value of current supplied to the heater and the current supply time without rotating the screw in the heating cylinder filled with resin, and the temperature of the heating cylinder detected by a temperature sensor is stored in advance as a reference. During actual molding, the temperature of the heating cylinder is detected at sampling periods while the quantity of heat generated by the heater is calculated, and based on the calculated quantity of heat, a difference (or ratio) between the stored reference and the detected cylinder temperature is calculated as a temperature rise attributable to shearing heat generated by the screw rotation (see, for example, JP 2001-225372A).
There has also been proposed a control method in which a screw back pressure model for metering, according to which a non-defective product could be produced, is obtained in advance and also a ratio model of the screw back pressure model, which indicates the ratio of the quantity of heat generated by the heater to the quantity of shearing heat, is obtained. During actual molding, the current supply to the heater for heating the heating cylinder is controlled such that the screw back pressure approaches the screw back pressure model, and at the same time the temperature of the heating cylinder is controlled such that the heat quantity ratio approaches the ratio model (see, for example, JP 2001-287255A).
The driving torque of the motor etc. for rotating the screw represents not only the load applied by the shearing of the resin but other loads. Thus, although the load applied to the screw as a whole can be monitored in terms of the driving torque, it is not possible to extract and monitor only the load applied by the shearing of the resin being melted.
Also, according to the aforementioned techniques disclosed in JP 2001-225372A and JP 2001-287255A in which the influence of screw rotation on shearing is detected, it is necessary to measure and store in advance the correspondence between heat generation by the heater, calculated from the value of current supplied to the heater and the current supply time without rotating the screw in the heating cylinder filled with resin, and the temperature of the heating cylinder detected by the temperature sensor. Thus, resin must be filled in the heating cylinder without rotating the screw, which makes it difficult to measure and store, as a reference, the correspondence between the heat generation by the heater and the temperature of the heating cylinder detected by the temperature sensor.